In-Situ Strain Measurements in Large Volumes of Hardening Epoxy Using Optical Backscatter Reflectometry

Abstract

Some large engineering structures are made by casting polymers into a mold. The structures can have complicated geometries and may be filled with other components, such as electrical transformers. This study investigated casting of large components made of epoxy. Epoxy is easy to pour, bonds well and has relatively low cure shrinkage. However, the cure shrinkage can lead to significant stresses or strains, causing large deformations that can lead to cracks.Understanding the curing process and related shrinkage is important for designing molds and controlling the production process. This study applied a new experimental method to measure strains due to cure shrinkage allowing many accurate local measurements along the length of an optical measurement fiber. The method is based on Optical Backscatter Reflectometry. Six distinct stages of the curing process can be identified. Previous measurements were limited to a few point measurements in small samples. This paper shows cure shrinkage in large samples and identifies some unexpected changes in behavior when going from small to large specimens. The behavior is explained qualitatively.

A Running Reference Analysis Method to Greatly Improve Optical Backscatter Reflectometry Strain Data from the Inside of Hardening and Shrinking Materials

“…The increasing temperature manifests itself as (apparent) positive strain readings. This rather experiment-specific feature is described in detail in [17]. It is important however to note that the fiber is not yet subject to a shrinking environment.…”

“…These were encapsulated in glass, have 10 kΩ resistance at 25 degrees Celsius and have a temperature range from −40-+250 degrees Celsius. See [17] for details of the temperature measurements setup. Initially, the optical fiber is surrounded by liquid epoxy.…”

“…At some point in the curing process, the epoxy will adhere to the optical fiber and be able to transfer forces and strain into the fiber. More details about the experimental setup and the stages of the curing process are presented in [17].…”

“…In addition, often, the sample size that can be used in such experiments is rather small. This may reflect an unrealistic picture of hardening processes in real-world technical applications, since the production and dissipation of heat or the diffusion of water out of a structure are size and probably geometry dependent [16,17].…”

“…The curing process is complex, being different at different locations inside a cast component, while the whole block exhibits an average shrinkage. This research is described in detail in [17]. Hence, the paper at hand does not focus on the curing characteristics, but on the challenges of measuring the strain distribution along the fiber inside the hardening material.…”